1. Field of the Invention
The present invention relates to a microscope apparatus for observing finely detailed objects such as semiconductor structures.
2. Description of the Related Art
A high-resolution scanning tunnel microscope (hereinafter referred to as an STM) has recently been developed for the observation of finely detailed objects. The STM has a resolution high enough to enable observation of, for example, atoms, and comprises a tip which is attached to a microscope body. For observation of a sample, the tip is placed approximately 1 .mu.m from the sample to be observed, so that electron clouds of the atoms of the distal end of the tip and those of the sample overlap one another, and a tunnel current is then caused to flow between the tip and the sample when a voltage is applied between them. Since the value of the tunnel current changes exponentially in accordance with the distance between the tip and the sample, therefore the STM utilizes the relationship between tunnel current and distance for observation of the sample. More specifically, the STM two-dimensionally scans the surface of the sample by means of the tip, measuring the size of the tunnel current at various measuring points on the sample surface. Based on the measured values of the tunnel current, the distance between the tip at each measuring point and the sample of observation is superaccurately measured, and the measured values of distance at the individual measuring points are plotted to obtain a three-dimensional image of the sample surface. However, in actual measurements, it is difficult to detect the distance between the tip and the sample with high accuracy. Generally, therefore, the tip is moved up and down for tracing the irregular configuration of the sample surface with two-dimentionally scanning the sample surface, so that the distance between the surface and the tip, and the tunnel current are constant. The three-dimensional image of the sample surface is obtained on the basis of the amount of the up-and-down motion of the tip.
The observation area of the above STM usually ranges from tens of nanometers square to about 1 .mu.m square. Thus, if the whole observation surface of the sample is substantially even, as in the case of observing regularly oriented atoms, for example, a desired observation image can be obtained at any region of the sample surface. In this case, therefore, the observation region need not be specially chosen. On the other hand, if the configuration of the observation surface is irregular, as in the case of observing the profile of a semiconductor device formed of a wafer carrying thereon a linear pattern 1 .mu.m wide and 0.4 .mu.m high, for example, the tip must be accurately located in a specified place for observation. However, the conventional STM cannot attain this.
Accordingly, an improved apparatus has been developed which combines an STM with a scanning electron microscope (SEM). However, since the SEM is designed for observation of a sample in a vacuum, this apparatus cannot be used for observation in the atmosphere or in water.
In another improved apparatus developed hitherto, an optical microscope is arranged diagonally behind an STM, and the tip of the STM is positioned at a desired observation region of the surface of a sample to be observed while the tip and the sample are being observed by the optical microscope. The optical microscope, however, is designed for diagonal observation of a sample from behind, so that it is difficult to obtain high magnifying power and it is difficult to accurately observe the relationship between the tip and the sample surface by the optical microscope. Thus, the tip cannot be accurately positioned with respect to the sample of observation.